More runners than ever before—from elites to midpackers—are talking about running technique and debating its importance. Which begs the question: Should you change your form? That depends...

In Flagstaff, McMillan broke Higgins's form-changing process into three phases. First, he had her focus on a series of nonrunning movements—one involved standing in place and lifting her knees one at a time, which was meant to engage the full range of movement and the full power of her hip muscles. In the second phase, Higgins did drills—high knees and lunges—hoping to take advantage of her new range of movement with dynamic, rather than static, exercises. In stage three, McMillan had Higgins run with a midfoot strike, higher knee lift, and a slightly longer stride. "Greg kept saying, 'Get an inch,'" Higgins said. To practice a cadence closer to 180 steps per minute, she clipped a metronome to her sports bra during training runs.

"We were able to see changes," McMillan told me. "We were able to get a longer stride, a slower cadence, and improve her basic speed. We timed her in 100 meters, and she got faster by a second, which is pretty significant." From the perspective of technique, the experiment was essentially a success. By September, Higgins was running the way McMillan imagined.

But it required a great deal of effort, both physical, as her body adjusted to a new stride, and cognitive—Higgins was conscious of every step she took, and she calculated that she was taking around 210,000 steps per week. Even on easy distance runs she felt sluggish. "It was so awkward," she said. "I was exhausted."

Struggling with easy distance runs, never mind hard workouts, is enough to wear down even the toughest runner, and eventually Higgins's mood soured. McMillan stopped assigning the more aggressive exercises. Then, in October, as Higgins was preparing to race the New York City Marathon, she landed awkwardly on a rock. A week later, she was diagnosed with a stress reaction in her left foot. Higgins believes that midfoot striking had weakened her metatarsals, small bones in the foot. "It was a buildup, where it was getting weaker and weaker," she says. (When I asked McMillan, he said he wasn't sure how she injured her foot. "She was training for a marathon. It's impossible to know for sure.") The next day, Higgins flew home to Colorado, where she moved into a one-bedroom apartment with her cat, Boston, and asked her brother-in-law, Mike Sharkey, to help coach her. She continues to tinker with her form, doing variations on exercises she previously did with McMillan, albeit cautiously. "I'm still working on [my form]," she says. "But I learned the hard way that being too aggressive can lead to injury."

It is possible to change form, as Harry Hollines and Paige Higgins prove. But is it practical? The answers, as Hollines and Higgins also suggest, are mixed. Even changing stride rate and length, which is likely the simplest form change to make, has proved challenging. "For distance running, people naturally select the stride length most efficient for them," Iain Hunter, Ph.D., a biomechanist at Brigham Young University, says. Hunter has examined what happens to running economy—a measure of how quickly the body burns oxygen—when runners deviate from their natural stride length. Even when a change should make a runner faster, like asking Higgins to take 180 steps per minute, Hunter has found that economy worsens. "We realized that changes [to form] will happen with training, but you don't make the changes and have the training follow," he says.